Molten salt carburizing process and replenishing salt for same

ABSTRACT

Carburizing of metals is effected by using molten cyanide baths containing activators. In the operation of such baths, due to consumption of the cyanide, it is necessary to add replenishing materials. The replenishing material is a mixture of activator and cyanide salt only, wherein the activator cyanide salt proportion is identical to that of the starting salt.

United States Patent Mueller [4 Jan. 18, 1972 [54] MOLTEN SALTCARBURIZING PROCESS AND REPLENISHING SALT 211 App], No.: 820,337

[52] US. Cl ..148/15.5,148/15,148/20 [51] Int.Cl ..C23c 9/12 [58] FieldoiSearch l48/l5, 15.5,20, 27, 28, 3O

[56] References Cited UNITED STATES PATENTS 1,970,852 8/1934Hardingetal, ..l48/15.5

2,819,992 1/1958 Albrecht ..148/30 X 3,117,038 1/1964 Hewson et al...148/15.5

OTHER PUBLICATIONS Nletals Handbook, 1948 Ed. pages 692- 696 Molten SaltBaths for Heat Treatment and Case Hardening of Steel," published byE. 1. du Pont de Nemours &' Co. May 7,1943,pgs.6 8, 25,26, 31 & 33.

Primary Examiner-Char1es N. Lovell AttorneyCullen, Sloman & Cantor 57 1ABSTRACT Carburizing of metals is efiected by using molten cyanide bathscontaining activators. 1n the operation of such baths, due toconsumption of the cyanide, it is necessary to add replenishingmaterials. The replenishing material is a mixture of activator andcyanide salt only, wherein the activator cyanide salt proportion isidentical to that of the starting salt.

1 Claims, N0 Drawings MOLTEN SALT CARBURIZING PROCESS AND REPLENISHINGSALT FOR SAME BRIEF DESCRIPTION OF THE INVENTION This invention relatesto the process of carburizing metals by means of molten cyanide bathscontaining activators. The invention relates more specifically to novelreplenishing-salts and novel replenishing steps in the process as awhole.

THE BACKGROUND OF THE ART It is known to carburize metal workpieces bythe use of molten cyanide baths containing activators. Known activatorsare alkali earth metal chlorides and chlorides known for such purposesare chlorides of barium and strontium.

It is also known to design a bath specification according to the desiredcarbon potential which is the point on the carbon content curve wherethe carbon content in relation to the depth of the carbon content fromthe surface is at a maximum.

It is also known that if a higher carbon potential is desired, a greaterproportion of activator salt in the bath should be maintained.

During the operation of a bath, not only does the liquid level of thebath decrease due to drag out losses and other losses but also thecyanide proportion decreases as the cyanide breaks down in thecarburizing process. Hence, it becomes necessary for the workmenpracticing the process to add replenishing materials.

The present practice prior to the invention is for the workmen to add tothe bath a desired quantity of cyanide salt from a stock of same, andadd also a desired or specifiedquantity of the activator, also from astock of the same. Great possibilities for variation have been observedand have plagued the industry and particularly the segment of theindustry which was anxious to provide an exact control of the carbonpotential and for this purpose to provide an exact control of the bathand particularly its replenishing steps.

Besides the method of replenishing two different types of salt, thereare also known baths that are replenished with just one salt. The saltfor the latter baths contains both cyanide and activator, but alsocontains diluents such as alkaline chlorides or carbonates. Theactivator content of this premixed salt is not equal to the activatorcontent to be maintained in the bath, so that with such baths it is notpossible to produce and maintain over long periods, an exactpredetermined surface carbon content.

Therefore, this invention airns in the direction of providing a moreexact control of the bath and particularly of the replenishing step andmaterials, with the view of eliminating mistakes and errors as much aspossible and to make the replenishing operation as exact and foolproofas possible.

THE INVENTION First, in the investigation of the operation of the bathwith a view to providing a better control for same came the discoverythat the carbon potential of the bath is exclusively dependent on theamount of activator contained in the bath. The activator is not subjectto decomposition during the operation of the bath. It is essential forthis discovery that the activator content be controlled within extremelyclose limits to produce and consistently maintain over long periods aspecified carbon potential.

Therefore, an effort was made to design a practical method ofreplenishing.

The first aspect of the improved replenishing method and material isthat the replenishing salt is supplied by a single stock of material sothat it was not necessary for the workmen to obtain replenishingmaterials from two stocks and to mix them. Hence, the first aspect ofthe novel replenishing salt is that it is a premix of the cyanide saltand the activator in predetermined proportions from which the workmanmerely draws as much as needed for replenishing.

The single replenishing premix includes both the activator and thecyanide salt.

The second aspect is based upon the knowledge that it is only thecyanide salt that is consumed by decomposition during carburizing. Weare not speaking here of reduction of liquid level due to drag outlosses but rather a change in the proportions of cyanide salt andactivator that occurs during the use of the process. The replenishingsalt of the invention is made up of the activator and cyanide'salt butthe novel aspect is that the percentage of activator with respect to thecyanide salt is exactly the same as the percentages of these twoingredients at the start of the carburizing operation and beforedecomposition of the cyanide salt.

It is known that in carburizing, the cyanide breaks down and there isformed within the bath a quantity of carbonate. The activator does notbreak down and remains constant. The novel replenishing salt includesonly cyanide salt and activator and these are in the proportions of thesame two ingredients at the start of the operation. This novel salt thenfor replenishing, maintains the bath in such a state that it returns thebath into the state desired to establish the desired carbon potential.

For example, let us consider a bath which at a specific period of itsoperation, and long after a previous period of operation is of thecontent of 10 percent cyanide salt, 20 percent activator salt, and 70percent carbonate. This bath is considered to have been designed asproviding a carbon potential of 0.5 and for this purpose was set up as abath of 20 percent activator and percent cyanide. During the long periodof operation and just prior to the specific period of operation, thebath has changed to, as stated, 10 percent cyanide, 20 percentactivator, and 70 percent carbonate.

At the end of the specific period of operation, let us say one day, thebathis found to have its cyanide content reduced to 7 percent and thecarbonate content increased to 73 percent but the activator content isthe same 20 percent.

Accordingly, for replenishing, I propose by this invention to use areplenishing salt of 20 parts activator and 80 percent cyanide and thisreturns the bath to its original condition but at all times maintainsthe bath at a not over 20 percent activator content.

While it is known that the constancy of carbon potential of a bath canbe predetermined and maintained by insuring a con stant prefixed levelof the activator, I consider as new the disclosure that despite the needfor replenishing, the constancy of the activator can be maintained byusing as a replenishing salt, a premix containing cyanide but whoseactivator content is exactly the same as the activator contentoriginally desired for the bath.

REPLENISHING FOR CHEMICAL DETERIORATION If the cyanide content hasdropped, I use as a replenishing mix a mixture of activator and cyanidesalt only, with the activator percentage being the same as that of thestarting salt.

REPLENISHING FOR VOLUME LOSS Considering a bath whose original contentis such that the activator level was predetermined according to thedesired carbon potential with the rest of the bath being cyanide andcarbonate, when the liquid level of the bath has dropped below thedesired level, replenishing is done here by a premix whose percentagesand content are the same as that of the starting salt, namely activatorplus cyanide plus carbonate. Thus, in the example given, replenishingfor volume loss would be done by a premix of 20 percent activator, 10percent cyanide salt, and 70 percent carbonate.

Contrast the foregoing with the replenishing salt which is used tooffset chemical deterioration. This replenishing salt would be withoutcarbonate and would be 20 percent activator and 80 percent cyanide saltin the example given.

FURTHER EXAMPLES I. In carburizing chains made up of links, I have usedas a starting salt a mixture of 20 percent barium chloride plus 10percent cyanide salt and 70 percent carbonate to insure a carbonpotential of 0.5. The salt I use for replenishing then is a premix of 20percent barium chloride and 80 percent cyanide.

2. In carburizing certain front wheel automobile drive components, suchas drive shafts and universal joints, 1 have operated to'insure a carbonpotential of 0.8. The starting mixture at the specific period ofoperation was set up as 35 parts barium chloride plus parts sodiumcyanide and 55 parts sodium carbonate. The replenishing salt forchemical deterioration is a premix of 35 parts barium chloride and 65parts sodium cyanide.

ADVANTAGES A. Salt consumption was reduced over a long time by the useof a premix formula. There was no tendency to add excess salt either byerror or inexperience or guesswork and personal variations and humanerrors did not affect the replenishing operation and consequently therewas no tendency to use more salt than was actually required forreplenishing.

B. This replenishing program provided a highly satisfactory control forthe level of retained Austenite. By using a controlled carbon potential,I enable the control of the amount of retained Austenite. Hence, usingthe process hereof it did not become necessary to harden by repeatedquenching and heating as was formerly the case to insure a predeterminedlevel of Austenite retention. Previously, because of uncertainties ofthe Austenite level, it was the practice to carburize and quench andthereafter to harden by reheating and quenching again. Because of thecertainty provided by my invention, it is necessary now only tocarburize and quench and there is no need for the additional steps ofhardening by reheating and quenching.

CONCLUSION Having described and stated all that I know of the processand salts hereof at the present time, I wish it understood that all ofthe foregoing is to be interpreted as illustrative only rather thanlimited and accordingly, I refer to the claims which follow for adetermination of the scope of the invention;

I claim: 1. In a molten cyanide salt carburizing process employing abath of the molten cyanide salt and an activator of the group consistingof barium chloride and strontium chloride; and wherein the bath atstarting of a first period of operation, has a predetermined ratio ofactivator to cyanide salt; and,

wherein the bath, at the end of that first period of operation, has thesame amount of activator, plus a reduced but satisfactory amount ofcyanide and an amount of carbonate formed by decomposition of thecyanide; and,

wherein the bath, at the end of a second period of operation has thesame amount of activator, plus a further reduced and now unsatisfactoryamount of cyanide and an increased amount of carbonate; and requiresrestoration to make up for the loss of cyanide in such second period ofoperation;

the step of restoring the bath after the second period of operation byadding a restoration salt premix of the cyanide salt and the activator,wherein the activator-cyanide salt proportion is identical to that atthe start of the first period of operation; and without carbonate.

